Regarding a gasoline engine, demand for reduction in fuel consumption is extremely great in terms of the reduction of CO2 or a great increase in gasoline price, and an attempt for improvement of fuel efficiency has been made using a technology such as lean combustion or exhaust gas recirculation. However, either one has a problem of defective ignition. In a spark plug used for a current gasoline engine, a high voltage pulse is applied between electrodes such that thermal plasma is generated by an arc discharge, and the fuel is ignited by the thermal plasma.
In contrast, practical use of a volumetrically high-efficient ignition method using low-temperature plasma has been proposed as a technology for improving the ignition stability. The low-temperature plasma refers to plasma in a non-equilibrium state where an electron temperature is high but an ion or neutral-particle temperature is low, and is characterized in that the low-temperature plasma, enables a multi-point simultaneous ignition which occupies a high, volume, that is, a volumetric ignition to be performed. By using the low-temperature plasma, it is possible to hinder consumption of the ignition plug, and because the production amount of radicals (active particles that are generated due to a discharge and serve as combustion initiation points) is large, it is possible to facilitate combustibility after ignition.
The low-temperature plasma is generated, by a barrier discharge, a corona discharge, a streamer discharge, or the like. Among them, the barrier discharge that is an alternating current discharge generated using a dielectric interposed between electrodes is a technique capable of stably generating the low-temperature plasma since a non-equilibrium discharge can be maintained over a wide electrode surface area.
In the barrier discharge, because thin-pillar-like minute streamer discharges are generated intermittently and evenly on an electrode surface, the low-temperature plasma can be generated uniformly in a wide range. On the other hand, because energy input by plasma, spreads throughout into the entire discharge space, input energy per unit, area is low. That is, although the barrier discharge may efficiently generate radicals, it can be said that the barrier discharge is a technique in which the radicals are uniformly distributed and tend to be diluted.
As the related art applying the barrier discharge to engine ignition, Patent Literature 1 proposes an ignition device in which an annular electrode is concentrically arranged outside a cylindrical dielectric electrode in which a rod-shaped center electrode is covered with a dielectric layer. In this example, the outer annular electrode is grounded and high-voltage alternating current waveforms are applied to the center electrode. Thus, the barrier discharge is caused to occur in a concentric electric field between the dielectric electrode and the annular electrode.